Boron trifluoride and a boron trifluoride complex constituted of boron trifluoride and a complexing agent (ligand) are well known as a so-called Friedel-Crafts catalyst, and the boron trifluoride complex has an excellent catalyst performance of suppressing side reactions and promoting only main reactions effectively as compared with AlCl3, FeCl3, sulfuric acid and the like. Therefore, various boron trifluoride complexes are industrially used widely as a catalyst in various chemical reactions such as alkylation, isomerization, polymerization, dimerization, condensation, addition, decomposition, dehydration, etc.
Main industrial use of the boron trifluoride or boron trifluoride complex includes, for example, a catalyst in producing ethyl benzene by gas phase alkylation from ethylene and benzene. The boron trifluoride or boron trifluoride complex is also used as a catalyst in the process for producing alkyl benzenes used for synthetic detergents or antioxidants by liquid phase alkylation reaction of an olefin and an aromatic compound.
Furthermore, the boron trifluoride or boron trifluoride complex is also used as a polymerization catalyst in producing petroleum resins and chroman-indene resins which are widely used in the fields such as adhesives and printing ink. When the boron trifluoride or boron trifluoride complex is used as the polymerization catalyst, it is possible to attain effects such as less deterioration in quality of a product and less corrosion of an equipment used. As mentioned above, the boron trifluoride or boron trifluoride complex provides a catalyst that can be used in various applications as a catalyst for production of various products in chemical industries.
The boron trifluoride complex constituted of boron trifluoride and a complexing agent (ligand) is used, depending on the reaction as intended, in the form of a complex in which various compounds are coordinated to boron trifluoride at an appropriate ratio. The boron trifluoride complex acts as a catalyst in the reaction as intended. It is general that the boron trifluoride complex is deactivated after stopping the reaction as intended to separate the boron trifluoride complex from the resulting reaction mixture. In order to separate the boron trifluoride complex from the reaction mixture, there are usually adopted a method of washing the reaction mixture with water after adding water to the reaction mixture to deactivate the boron trifluoride complex therein, a method of washing the reaction mixture with water after neutralizing the reaction mixture solution by a basic aqueous solution of ammonia, sodium hydroxide, lime or the like.
However, in the above methods, a waste water containing a hydrate of boron trifluoride or a neutralized product of boron trifluoride in a high concentration is discharged. Therefore, in recent years, there is a demand for suitable measures to be taken for removal of the waste water containing fluorides or boron in consideration of the problem of environmental pollution. In particular, since it is difficult to remove boron easily by a current technology for waste-water treatment, and since it is costly to remove boron completely, there is a demand for removal of boron at low costs. Furthermore, since boron trifluoride is expensive, there is a demand for recovery and reuse of the removed boron trifluoride or boron trifluoride complex.
However, for example, in the condensation reaction of olefins using the boron trifluoride complex as a catalyst, the boron trifluoride complex is almost dissolved in the reaction mixture or forms an emulsion. Hence, even if the reaction mixture is allowed to stand, it is difficult to completely separate the reaction mixture and the boron trifluoride complex catalyst from each other. Therefore, the boron trifluoride complex is removed from the reaction mixture by washing the reaction mixture with water or the basic aqueous solution. However, if water or the basic aqueous solution is added to the boron trifluoride complex, the boron trifluoride catalyst forms a water complex such as BF3(H2O)n or a boron trifluoride salt, and cannot be repeatedly used as a catalyst as it is. Also, there is known a method of separating a catalyst liquid layer from the reaction mixture using a complex of BF3 with phosphoric acid, acetic acid or phenol (for example, refer to Patent Document 1). However, the above reaction proceeds only slowly unless the catalyst concentration is 10% or more. Moreover, since the catalyst acts as a Broensted acid, side reactions such as isomerization tend to occur, and consequently the aforementioned separation method cannot be adopted particularly for a condensation dimerization reaction of olefins.
Conventionally, there is known a method for the recovery of boron trifluoride or a boron trifluoride complex, wherein the reaction mixture solution containing the boron trifluoride or boron trifluoride complex is contacted with calcium fluoride (CaF2) at a temperature of 200° C. or lower, and the resulting calcium tetrafluoroborate (Ca(BF4)2) is heated at a temperature between 100° C. and 600° C., to obtain the boron trifluoride to be recovered and calcium fluoride (for example, refer to Patent Document 2).
Similarly, there is known a method for the recovery of boron trifluoride by contacting a fluoride such as lithium fluoride, strontium fluoride and barium fluoride with the reaction mixture solution containing the boron trifluoride or boron trifluoride complex to produce a tetrafluoroborate and heating the tetrafluoroborate at a temperature between 100° C. and 600° C. (for example, refer to Patent Document 3, Patent Document 4 and Patent Document 5).
However, in the reaction using the boron trifluoride or boron trifluoride complex as a catalyst, it is necessary to react at a low temperature below room temperature in many cases. Moreover, when the temperature is raised to about 100° C. after stopping the reaction, at which the tetrafluoroborate is synthesized advantageously, a side reaction tends to occur, which results in decrease in the yield of the reaction product as aimed or deterioration in quality thereof. The tetrafluoroboric acid is hardly formed below room temperature. In addition, decomposition reaction by heating at high temperature is not desirable from the viewpoint of energy saving.
Moreover, there is a problem that, when the reaction mixture solution containing the boron trifluoride or boron trifluoride complex is viscous, much time and effort are required in order to separate borates such as calcium tetrafluoroborate from the reaction mixture.
There is also known a method for the recovery of the boron trifluoride or boron trifluoride complex, wherein the boron trifluoride or boron trifluoride complex is precipitated and separated from a nonconducting fluid by applying d.c. voltage and/or a.c. voltage to the nonconducting fluid in which boron trifluoride is dispersed and/or dissolved, and then the boron trifluoride or boron trifluoride complex thus separated is heated (for example, refer to Patent Document 6).
However, in the recovery method, it is necessary to use an electrical facility capable of continuously applying a voltage of several hundreds of volts from an external power supply for a time period of 30 minutes or longer after stopping the reaction. Since a side reaction tends to occur owing to the application of voltage, a process for stopping the reaction is required. There is also a problem that the boron trifluoride or boron trifluoride complex dissolved in the reaction mixture is not always separated completely by the application of voltage.
Furthermore, there is disclosed a method of recovering boron trifluoride or a boron trifluoride complex using a hydrofluorocarbon compound, etc., and reusing the recovered boron trifluoride or boron trifluoride complex (for example, refer to Patent Documents 7 and 8). In the method, there can be attained a good recovery rate of the boron trifluoride or boron trifluoride complex. However, there remains such a problem that the hydrofluorocarbon used is expensive. In addition, since organic substances are dissolved in the hydrofluorocarbon, it is necessary to additionally conduct the step of removing the organic substances from the hydrofluorocarbon.